Peterson's pledge to vote for Sanders is not for the primaries — his state has already voted — but at the Democratic National Convention in July. As a member of Congress, he's automatically a superdelegate, which means he's free to vote as he wishes to at the convention. But despite his own political inclinations, he's aligning himself with the preference of his constituents, who voted overwhelmingly in favor of Sanders.

As the Huffington Post notes, Peterson is one of the most conservative Democrats in Congress and a co-founder of the Blue Dog Coalition, a group of moderate Democrats that skew conservative on fiscal issues like taxation and regulation.

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Peterson said he's not yet sure if he'll attend the convention, but that if he thinks his vote makes a difference in the final delegate count, he'll be there.

Currently Hillary Clinton has the support of almost all of the superdelegates who have announced whom they plan to vote for, although their pledges are not set in stone and could change, especially in the unlikely event that Sanders takes a decisive lead in the race for state delegates.